This invention is based upon the general problem of simplifying the known preparation of wash-active .alpha.-sulfofatty acid esters from fats and oils, particularly those of natural origin, in such a way that the fatty acids or fatty acid mixtures of vegetable and/or mineral origin can be used as a feasible alternative in the production of modern detergent compositions, thus making it reproducible for large scale technical purposes. In the field of washing and cleaning agents, particularly fabric detergents, the emphasis today is, as is known, on petroleum-based synthetic products although it has been known for decades that high-grade wash-active components can also be obtained from, in particular, the fatty acid triglycerides of natural starting materials.
Thus, U.S. Pat. No. 2,195,187, incorporated herein by reference, describes .alpha.-sulfofatty acids and their esters as wash-active substances. They are obtained by sulfonation of lower alkyl esters of saturated higher fatty acids with sulfur trioxide. The lower fatty acid alkyl esters used as starting materials are obtained by re-esterification of hydrogenated fats and oils with monovalent lower alkanols, particularly methanol.
Scientists of Henkel KGaA have dealt intensively in later studies with this class of wash-active .alpha.-sulfofatty acids and corresponding fatty acid esters, as well as their salts. U.S. Pat. No. 3,256,303, incorporated herein by reference, for example, describes a process for the production of this class of compounds. Fatty acids and fatty acid esters, which contain from 6 to 28 carbon atoms in the fatty acid radical, which have no other sulfonatable or sulfatizable groups, apart from the .alpha.-position carbon atom of the fatty acid radical, and which have an iodine number less than 5, are sulfonated with a sulfur trioxide-inert gas mixture, and the reaction product is neutralized. A parallel process for the production of the same compounds, working with alternate process conditions but at the end with the same means, is described in U.S. Pat. No. 3,158,632, incorporated herein by reference.
One of the main difficulties in this field is the color-instability of the fatty acid-containing starting material in the sulfonation stage. Dark black-brown crude products are obtained, which products must be worked up to light-colored products for use in washing and cleaning agents. The color of the crude sulfonation products depends to a certain extent on the working conditions. However, the technical utilization of this interesting possibility of the raw material is prevented by the following fact: the higher the yield in the sulfonation stage is, the darker is the reaction product and the greater are the difficulties of obtaining light-colored end products.
To reduce this difficulty, and to in particular improve the control of the sulfonation reaction while avoiding overheating, which can lead to undesired discolorations and secondary reaction, German Published Application (DOS) No. 12 62 265, incorporated herein by reference, suggests the sulfonation of mixtures of from 25 to 95 percent by weight of alkyl benzenes and from 5 to 75 percent by weight of saturated fatty acid esters. Here considerable quantities of alkyl benzenes used as reactive diluters are supposed to overcome the color difficulties.
The importance of the constitution of the fatty acids or fatty acid mixtures to be sulfonated is well known in the art. It is particularly required that the fatty acids to be sulfonated in .alpha.-position contain no, or only a few, double bonds, as well as no other reactive groups--especially no hydroxyl groups. By selection of suitable oils or fats, this problem is confined to the elimination of unsaturated bonds in the fatty acid molecule. These defects are eliminated by extensive hydrogenation of the starting material prior to sulfonation. The literature of the state of the art requires iodine numbers less than 5, preferably less than 2. In practice, far lower iodine-numbers, for example, in the range of from about 0.1 to 0.3, are used.
The separation of interfering accompanying substances from the fatty acids of fatty acid esters to be sulfonated is likewise required to reduce the discoloration problem. With respect to fatty acids and fatty acid esters used as a starting material, it is recommended to start from distillates (see, for example, U.S. Pat. No. 3,158,632, Col. 3, second paragraph). If distillation is not possible, for example, with triglycerides, other purifications steps are recommended for removing albuminous and slimy substances.
Bleaching of the crude sulfonic acid derivatives has always been required as the last step. Two particular methods are generally known to the art: acid bleaching with hydrogen peroxide (see, U.S. Pat. No. 3,159,657, incorporated herein by reference), and combination bleaching, where an acid hydrogen peroxide bleaching stage is followed by neutralization of the sulfonic and partly bleached material, followed again by bleaching with hydrogen peroxide, or better with hypochlorite (see, U.S. Pat. No. 3,452,064, incorporated herein by reference).
Special difficulties or problems, regarding discolorations occur when the sulfonation is to be increased to yields of over 90% or even to sulfonation degrees of over 95%, which problems are dealt with extensively in U.S. Pat. No. 3,485,856, incorporated herein by reference. According to this patent sulfur trioxide has a highly disintegrating effect on saturated fatty acid esters which are free of alcoholic hydroxyl groups, which leads unavoidably to deep-dark discolored sulfonation products in the production of highly sulfonated products with a degree of sulfonation of at least 90%, preferably at least 94%, and particularly at least 96%.
The increase in the degree of sulfonation in these ranges is not only of interest for economic reasons, however, since other factors require such high degrees of sulfonation too. Ester sulfonates with a corresponding low degree of sulfonation lead to difficulties in the conventional production of detergent compositions by spray drying. High pluming values appear in the processing of these ester sulfonates. Furthermore, the degree of sulfonation of ester sulfonates is directly related to an undesired by-product formed in this reaction, namely, .alpha.-sulfofatty acid. This compound, which is present after neutralization as a disodium salt, is poorly water-soluble and is therefore unsuitable as a raw material for detergents. Increasing the degree of sulfonation from 90% to 96% in these ester sulfonates causes, for example, a decrease of this undesired by-product from 25% to 16%.
U.S. Pat. No. 3,485,856, which deals with the last-mentioned problem, suggests that to limit the discoloration and to maintain certain temperatures in the sulfonation reaction, water should be introduced into the sulfonation product in such quantities that sulfuric acid is formed from the existing excess sulfur trioxide and the water, the concentration of the H.sub.2 SO.sub.4 being in the range of from about 20 to 100 percent by weight at the start of the following bleaching phase.
For the large-scale technical process, however, new difficulties arise, which represent a considerable risk source. The viscosity of the sulfonation product is greatly influenced in the high acid range by even very small amounts of water. The addition of 2% hydrogen peroxide in the form of a 35% solution--with the required amounts of water--to the crude sulfonation product with a C.sub.16 /C.sub.18 -chain length, leads to a sharp viscosity rise. In the continuous technical process, this involves the risk that the pipe line will be clogged. This viscosity rise is particularly critical with an addition of from 1.8 to 2.5 percent by weight of hydrogen peroxide, based upon the weight of the crude sulfonic acid.
This characteristic of the crude sulfonating product resulting in technical difficulties is also the reason that bleaching with hydrogen peroxide leads to considerable difficulties in the acid range, at least in fatty acid alkyl esters with C.sub.16 /C.sub.18 -chain lengths, as they can be obtained from, for example, tallow and palm oil. If fatty acid methyl esters, obtained by re-esterification or esterification, are sulfonated for from about 10 to 90 minutes at temperatures of from about 70.degree. to 130.degree. C., about 95% of deep-black reaction products are obtained at the desired degrees of sulfonation. The use of these products as a raw material for detergents requires bleaching. Bleaching in the alkaline medium with hydrogen peroxide or sodium hypochlorite does not lead to satisfactory results. Even sodium hypochlorite in amounts of 3 percent by weight--based upon the weight of wash-active substance--leads only to Klett dye numbers in the range of 140. Larger amounts of sodium hypochlorite can no longer be used, however, since the sodium chloride formed in this bleaching reaction leads to unacceptable thickening of the neutralized ester sulfonate paste. Nor are Klett dye numbers in the purified product which are over 50 to 60 acceptable.Incompletely bleached ester sulfonates give rise to the risk of becoming darker. Color constancy is, however, absolutely necessary for spray-drying when this surfactant is used in detergents.
The best bleaching method for crude ester sulfonates so far is the use of hydrogen peroxide in a highly acid range (pH=0), the bleaching effect being particularly pronounced. However, such a procedure involves the risk of the above-described sudden viscosity increase. With highly sulfonated ester sulfonates, even 2 percent by weight of hydrogen peroxide is not enough to bring the Klett dye number to 50.
After neutralization of the crude sulfonic acid, it is therefore necessary to bleach again with sodium hypochlorite. A reduction in the amount of the bleaching agent with a simultaneous increase of the bleaching time leads to less favorable colors. The use of hydrogen peroxide in large quantities has, in addition, the effect that bleaching with from 0.5 to 1 percent by weight of sodium hypochlorite can only be possible after the decomposition of the hydrogen peroxide which had not reacted in the bleaching reaction, which takes about 24 hours. If hydrogen peroxide is still present, sodium hypochlorite has a greatly reduced bleaching effect.
There are also a number of other difficulties. Due to the great viscosity increase with the addition of 2 percent by weight of hydrogen peroxide, for example, to the crude sulfonic acid, it is not possible to obtain paste concentrations higher than 28 percent by weight of wash-active substance. In this bleaching method there are also problems with foaming, which are technically difficult to control, in particular, the foam introduced into the crude sulfonic acid leads to a further viscosity increase.
The many difficulties appearing in the various stages of the total process lead according to our present knowledge to a forced compromise between sulfonation and bleaching. The optimum degree of sulfonation obtainable in practice are about 90%.